Conventionally, with regard to engines to carry out lean combustion, such as a diesel engine, it is concerned that a relatively large amount of nitrogen oxides (hereinafter referred to as NOx) is discharged. As its measures, it has been known to provide an exhaust gas recirculation system (EGR: Exhaust Gas Recirculation) to recirculate part of exhaust gas into an intake path (for example, see patent document 1).
The EGR system includes an EGR path that communicates an exhaust path and an intake path of an engine with one another, and an EGR valve provided in the EGR path. Then, an opening degree of the EGR valve is adjusted so as to regulate the amount of exhaust gas (the amount of EGR gas) recirculated from the exhaust path to the intake path via the EGR path, and an EGR ratio during intake air is set to a target EGR ratio set in advance. Thus, part of the exhaust gas is recirculated to the intake path, which allows a combustion temperature in the combustion chamber to drop and restrains the generation of NOx so as to improve the exhaust emission.
Also, in general, in the EGR system of this sort, an EGR cooler is provided in the EGR path. For example, in a system including only one (one system) EGR mechanism, which is disclosed by the patent document 1, the EGR cooler is applied to the EGR path of the EGR mechanism.
Also, as is disclosed in patent document 2, in a system (hereinafter referred to as “MPL-EGR system”) including a high pressure EGR mechanism (hereinafter referred to as “HPL-EGR mechanism”) and a low pressure EGR mechanism (hereinafter referred to as “LPL-EGR mechanism”), the EGR cooler is applied to only the EGR path of the LPL-EGR mechanism.
The HPL (High Pressure Loop)-EGR mechanism is such that the exhaust gas is recirculated from the exhaust path on the upstream side with respect to a turbine of a turbocharger (for example, exhaust manifold) to the intake path on the downstream side with respect to a compressor of the turbocharger.
Also, LPL (Low Pressure Loop)-EGR mechanism is such that the exhaust gas is recirculated from the exhaust path on the downstream side with respect to the turbine of the turbocharger to the intake path on the upstream side with respect to the compressor of the turbocharger. Accordingly, in the LPL-EGR mechanism, the exhaust gas can be recirculated to the intake air prior to supercharging by the compressor (intake air area under relatively low pressure), so that the recirculation amount can drastically be increased, and remarkable effects can be achieved in terms of the improvement of the exhaust emission.
Also, as a form of use of the MPL (Middle Pressure Loop)-EGR system, as disclosed in patent document 3, in a low load operating area of an engine, the exhaust gas having a relatively high temperature is recirculated using the HPL-EGR mechanism only so as to achieve the stability of combustion and restrain the emission of HC or CO. Also, in a high load operating area of the engine, the exhaust gas having a relatively low temperature is recirculated using the LPL-EGR mechanism only so as to restrain the generation of smoke accompanied with a high temperature of the intake air. Also, in a moderate load operating area of the engine, the exhaust gas is recirculated using both the HPL-EGR mechanism and the LPL-EGR mechanism so as to restrain the generation of HC, CO, and smoke.
Accordingly, the EGR cooler is not applied to the HPL-EGR mechanism that is aimed at recirculating the exhaust gas having a relatively high temperature, but the EGR cooler is applied to only the LPL-EGR mechanism that is aimed at recirculating the exhaust gas having a relatively low temperature.